How can I add properties to a class using a decorator that takes a list of names as argument?

Question

I would like to add many dummy-properties to a class via a decorator, like this:

def addAttrs(attr_names):
  def deco(cls):
    for attr_name in attr_names:
      def getAttr(self):
        return getattr(self, "_" + attr_name)
      def setAttr(self, value):
        setattr(self, "_" + attr_name, value)
      prop = property(getAttr, setAttr)
      setattr(cls, attr_name, prop)
      setattr(cls, "_" + attr_name, None) # Default value for that attribute
    return cls
  return deco

@addAttrs(['x', 'y'])
class MyClass(object):
  pass

Unfortunately, the decoarator seems to keep the reference of attr_name instead of its content. Therefore, MyClass.x and MyClass.y access both MyClass._y:

a = MyClass()
a.x = 5
print a._x, a._y
>>> None, 5
a.y = 8
print a._x, a._y
>>> None, 8

What do I have to change to get the expected behavior?

Answer 1

You almost had it working. There is just one nit. When creating the inner functions, bind the current value of attr_name into the getter and setter functions:

def addAttrs(attr_names):
  def deco(cls):
    for attr_name in attr_names:
      def getAttr(self, attr_name=attr_name):
        return getattr(self, "_" + attr_name)
      def setAttr(self, value, attr_name=attr_name):
        setattr(self, "_" + attr_name, value)
      prop = property(getAttr, setAttr)
      setattr(cls, attr_name, prop)
      setattr(cls, "_" + attr_name, None) # Default value for that attribute
    return cls
  return deco

@addAttrs(['x', 'y'])
class MyClass(object):
  pass

This produces the expected result:

>>> a = MyClass()
>>> a.x = 5
>>> print a._x, a._y
5 None
>>> a.y = 8
>>> print a._x, a._y
5 8

Hope this helps. Happy decorating :-)